An example of prior art optical machining apparatus disclosed in Japanese Patent Kokai Publication No. 220991/1988 is illustrated in FIG. 1 and FIG. 2. FIG. 1 is a schematic diagram of the apparatus, and FIG. 2 is a plan view of the mask showing an example of the pattern of the mask.
As illustrated in these figures, a light source, which is in the form of an KrF excimer laser oscillator 101, emits a laser beam 107 of a wavelength of 248 nm. The laser oscillator 101 comprises a full-reflecting mirror 102, a laser output mirror 103 having a certain light transmission coefficient t (&lt;1), and laser medium 104 filled between the mirrors 102 and 103. A mask 105 is provided with apertures having a pattern corresponding to the pattern which is desired to be created on a workpiece 109 by the optical machining.
The light beam 107 which is emitted from the light source 101 is expanded by a beam expander 106, and part of the light beam 107 which is incident at the aperture pattern on the mask 105 is passed through the mask 105.
The mask 105 comprises a transparent plate 105a formed of a synthetic quartz which permits passage of light and light nontransmissive parts 105c formed to leave light-transmissive parts 105b of a certain pattern. The light from the light source 101 is passed through the transparent parts 105a.
In FIG. 2, the pattern 105b is shown to be continuous. But this is for convenience of illustration. Actually, the pattern 105b is formed of a multiplicity of light-transmissive windows of about 20 .mu.m in diameter arranged to form the pattern 105b. About 100 such light-transmissive windows are present per 1 cm.sup.2. The aperture proportion (the ratio of the area of the pattern 3b forming the light-transmissive parts to the entire mask area) is about 0.03%.
An image-forming lens 110 having a focal length F is provided, being separated from the mask 105 by a distance A. A printed wiring board of polyimide which is a workpiece to be machined is placed at a distance B from the image-forming lens 4.
By the action of the image-forming lens 110, the part of the laser beam, 111, that is passed through the aperture pattern 105b of the mask 105 is passed through an image-forming optical system 110, and is image-formed on the surface of the workpiece 109.
Operation will now be described. The laser beam generated by oscillation of the laser oscillator 101 and is emitted through the laser output mirror 103, as indicated by reference numeral 107, is expanded by the expander 106 to the size of the area of the mask 105, and is irradiated on the mask 105. The mask 105 has a certain pattern 105b shown in FIG. 2 where it permits passage of the laser beam 107. The laser beam 107 is thereafter incident on the image-forming lens 110, and is projected on the workpiece 109 if the relationship: EQU 1/A+1/B=1/F
holds. The image as projected on the workpiece 109 is an inversion of the pattern 105b. The machining of through-holes, such as via-holes is thereby effected on the workpiece 109 in accordance with the pattern. The magnification factor of the projected image relative to the pattern 105b on the mask 105 is B/A.
The aperture proportion of the mask 105 is as small as 0.03%, and the rest of the light, which amounts to 99.97% of the total incident light, is absorbed or reflected by the mask 105.
As the prior art laser machining apparatus configured as described above, the laser beam is emitted through the semi-transparent film, the power density is low. Moreover, the laser beam irradiated on the mask is mostly absorbed or reflected by the mask magneto-optic information recording medium 105, the most (99.97%, in the above example) of the energy of the laser beam is not actually utilized for the machining but is wasted, and the utilization factor of light is low.